The prior art is rich with approaches to measurement of bone characteristics using acoustic and other methods with a view to identifying patients in need of treatment for osteoporosis. Many acoustic techniques utilize a first transducer to provide an acoustic signal, typically at ultrasonic frequencies, to the subject from a first external location and a second transducer at a second external location disposed on the opposite side of the bone of interest to receive the signal transmitted by the first transducer through the bone and intervening soft tissue. (The transducers are typically coupled to the subject through a suitable fluid, such as water.) Under one approach, there is determined the rate of Broadband Ultrasound Attenuation (BUA) in the range of approximately 300 to 700 kHz. The BUA is determined by measurement of the attenuation at a plurality of frequencies and then fitting the measurements to a suitable linear logarithmic-amplitude versus frequency scale. However, as an indicator of osteoporotic bone, BUA does not provide a desirable level of specificity and sensitivity.
A survey of prior art bone characteristic measurement is provided in the excerpt, from a document prepared by Merck & Co., Inc., which is submitted herewith and incorporated herein by reference as background material. (The discussion in this document of the Osteo Sciences Ultrasonometer is not prior art.)
The present invention provides, in some embodiments, enhanced specificity and sensitivity in detecting osteoporotic bone by utilizing a number of surprising discoveries, including: (i) the attenuation of ultrasound in bone is nonlinear, and utilization of received signal information that is lost in BUA analysis permits more accurate assessment of bone condition; and (ii) significant portions of the acoustic energy lost in attenuation in direct transmission through the bone can be measured by suitable placement of a third transducer in a position distinct from the path of direct transmission.
In accordance with a preferred embodiment of the invention, there is provided an apparatus for externally measuring in a vertebrate subject the extent of osteoporosis of a bone. The embodiment has first and second transducers and a mounting arrangement for mounting the transducers in spaced relationship with respect to the bone. A signal generator, in communication with the first transducer, causes the first transducer to produce acoustic signals, having energy distributed over a frequency range, that are propagated into the subject and received by the second transducer along a path that includes the bone. Finally, the embodiment has a signal processor, in communication with the second transducer, for providing a measurment that is a function of at least one of spectral or temporal components of the signal received by the second transducer. The function is selected for its ability to minimize differences among successive measurements taken of the same individual and to maximize differences in measurements taken of different individuals, so that the measurement relates to the extent of osteoporosis of the bone. In a further embodiment, the function is a weighted sum of spectral components of the signal received by the second transducer, and the weights are selected for their ability to minimize differences among successive measurements taken of the same individual and to maximize differences in measurements taken of different individuals.